Propylene polymer production processes employing catalysts comprising metallocene compounds with substituted indenyl ligands are described in Japanese Patent Kokai H6-184179, Japanese Patent Kokai H6-100579 and Japanese Patent Kokai H7-188318.
These metallocene compound-containing catalysts, however, are generally costly. It has been a desired goal to further improve the polymerization activity of such catalysts in order to reduce the production cost for propylene polymers obtained using them.
Metallocene compounds of this type have been developed mainly with the aim of allowing production of high molecular weight propylene polymers and permitting a high degree of control over the stereoregularity of the resulting propylene polymers, but the desired performance has not yet been achieved to a satisfactory extent.
As has been documented by T. Sugano in SPO '99 (1999), pp.31–53, production of propylene/ethylene copolymer using a metallocene compound-containing catalyst usually results in a major decrease in molecular weight of the copolymer product as the ethylene unit content increases. A demand has existed, therefore, for development of a metallocene compound-containing catalyst which allows production of propylene/ethylene copolymer of sufficiently high molecular weight even when the ethylene unit content is high.